1. Field of the Invention
The present invention relates to a thermoformable multilayer film or sheet superior in gas barrier properties, thermoformability, mechanical properties, and appearance, to a thermoformed container produced therefrom, and to a resin composition for production thereof.
2. Description of the Related Art
Ethylene-vinyl alcohol copolymer (EVOH) is used as a good packaging material for foods and medicines whose quality retention is important. Packaging containers using EVOH are available in various types. One of the most common examples among them are thermoformed containers produced from multilayer films or sheets having an EVOH layers. Such film or sheet is also used for packaging without thermoforming.
The conventional packaging technology of thermoformed containers made from films has been developed to keep lean meat, processed meat, ham, sausage, etc. in good quality until their consumption. This object is achieved by keeping them away from oxygen during their storage, because oxygen deteriorates their quality. One package design for this end is to place contents in a pouch made of laminated film comprising biaxially-oriented layers having improved strength and barrier properties or to tightly enclose contents in oriented film by means of the heat shrink technology.
Another package design which has prevailed recently is an air-assisted or plug-assisted thermoformed film container with its top sealed with film. The advantage of this package design is that contents fit the container and are readily visible to consumers. In addition, it permits a label to be pasted to any desired position. (This is convenient for sales conforming to the recently enforced PL law.) Moreover, such containers are easy to display on the shelf if they are of uniform shape.
The multilayer film suitable for such thermoformed containers having low oxygen permeability is made from polyvinylidene chloride, for instance. Containing halogen, this plastic is not desirable from the standpoint of environmental protection for the use of packaging material. Another example of the multilayer film is one which has a nylon layer for good oxygen barrier properties and thermoformability. Nevertheless, it is still required to improve the oxygen barrier properties in order to meet consumers' demand for extended storage life.
EVOH is known as a material which has good oxygen barrier properties and causes environmental problems only slightly. However, it is liable to extreme thinning at corners and streaking and wrinkling at the time of thermoforming. These defects lead to a decrease in impact resistance. In addition, EVOH film tends to burst or give dull corners (due to lack of close contact with the mold) during thermoforming. This problem is more serious in the case of thin laminate film than in the case of thick laminate sheet.
Although no concrete means has been reported to address the above-mentioned problem involved in thermoforming, there are some analogous ones which resort to lamination of the EVOH layer with nylon or blending of EVOH with polyolefin and thermoplastic polymer containing carbonyl groups (as reported in U.S. Pat. No. 4,079,850). However, combination of EVOH and nylon or other polymers has an adverse effect on gas barrier properties, thermal stability (during film production), and clarity (depending on the dispersion of the resin blended).
A conventional application of comparatively thick sheet having an EVOH layer is to thermoformed containers for food packaging. Nowadays foods are sold in containers (such as cups and trays) which permit consumers to enjoy foods without dishing them up after purchase. Typical examples are containers for jelly, custard pudding, yogurt, and juice. Some of such containers are designed for storage by consumers without the necessity of transferring contents into ordinary containers after purchase (such as cups for miso).
Thermoformed containers for such food packaging as mentioned above need good form stability and oxygen barrier properties so that they keep contents in good quality. The first object is achieved by using polypropylene (or propylene homopolymer) which has balanced stiffness and impact strength. The second object is achieved by using a barrier layer of EVOH (to protect contents from deterioration by oxidation).
When made from a laminate sheet consisting of inner and outer layers of polypropylene and an intermediate layer of EVOH, the resulting thermoformed container would be poor in clarity (making contents less visible) and also poor in impact resistance as a whole (because of the propylene and EVOH layers lacking impact resistance). Moreover, it would have cracks or wavy patterns on its side walls (and hence look poor) because of EVOH's poor thermoformability. In order to improve thermoformability and impact resistance, there have been proposed several methods, including incorporation of nylon into EVOH. However, they still involve problems with unsatisfactory thermoformability, insufficient gas barrier properties, low thermal stability (at the time of sheet production), and decreased clarity (due to incomplete resin dispersion).
The recent widening application area requires thermoformed containers to have good clarity (for contents visibility) and good impact resistance (against dropping) and also to have an intricate shape (formed by deep drawing). A conceivable way to improve clarity is to make the inner and outer layers from propylene random copolymer, which has good clarity. A conceivable way to improve impact resistance is to make the inner and outer layers from propylene block copolymer, which has good impact resistance. Unfortunately, both propylene copolymers have a lower thermoforming temperature than propylene homopolymer, and EVOH (for the barrier layer) is poor in thermoformability at such a temperature (resulting in cracking and wavy patterns).
On the other hand, containers for custard pudding and jelly are made from polystyrene, which is superior in form retention and gloss. For some kinds of contents (such as fruit jelly) which need flavor retention, thermoformed containers made from a polystyrene-EVOH laminate are used. This laminate is poor in thermoformability as in the case of the above-mentioned propylene copolymer because polystyrene has a lower thermoforming temperature than propylene homopolymer.